Conventional continuous-flow heaters have a housing provided with at least one flow channel for a fluid to be heated. A panel heating element lies flush against a surface of the housing.
A heater of this type is disclosed in U.S. Pat. No. 4,866,250 where a metallic housing is circumferentially surrounded by a heating foil. The heating foil is enclosed by a thin metallic foil and, in turn, an insulating wall of foamed material or a similar insulating substance is applied to the metallic foil. The entire assembly is accommodated in an outer second housing which can be covered by insulating layer. This known continuous-flow heater serves to preheat liquid fuels.
Continuous-flow heaters are also used to heat lacquers of high viscosity. These lacquers can then be processed with reduced additions of thinners or solvents. Furthermore, the decrease in viscosity due to heating makes it possible to obtain a fine dispersion at lower spray pressures.
When such lacquers are heated, they can produce vapors capable of exploding upon ignition. Since explosions are obviously dangerous, measures must be taken to avoid explosions entirely or to prevent these from doing damage.
This safety aspect is not dealt with in U.S. Pat. No. 4,866,520. Possibly, it is assumed that danger can be avoided by embedding the heating element in other materials. However, embedment has the drawback that repair of the heater becomes very costly. In many cases, exchange or repair of the heating element cannot be accomplished without the destruction of other components.
Also known are continuous-flow heaters having cartridge heating elements or heating coils between the flow channels. The cartridge elements or heating coils are either cast into the housing or located in bores which run between the flow channels. The fundamental design of such heaters is the same as that of the heater disclosed in U.S. Pat. No. 4,866,250. The cartridge elements or heating coils are completely enclosed by the housing so that vapors from lacquers or other fluids being heated cannot travel to the cartridge elements or the heating coils to be ignited. Repair of the heater, e.g., following failure of a cartridge element or heating coil, is virtually impossible even when the cartridge elements or heating coils are located in bores. Thus, after an extended operating period, removal of the cartridge elements or heating coils from the bores becomes extremely difficult.